This invention relates to near field antenna measurement systems generally and, more particularly, to novel systems and methods which allow a monitor in the near field to receive a signal approximating that which the monitor would receive if located in the far field. The antenna measurement systems are also usable as plane wave sources and in indoor antenna test ranges.
While the present invention is described as being especially useful with near field antenna monitors for microwave landing systems (MLS) for aircraft, it will be understood by those skilled in the art that it has other useful applications as well.
Part of a typical MLS is concerned with angle guidance for aircraft, which is accomplished through the use of two electronically scanned microwave antenna arrays--one for azimuth and the other for elevation. Distance is measured by a third element not of concern here. Because of the criticality of the application, each antenna is continuously monitored in the field in real time to measure the quality of the microwave beam. Ideally, the beam should be monitored in the far field where it is focused, that is, a distance from the antenna of at least 2D.sup.2 /.lambda., wherein D=aperture of the transmitting device and .lambda.=wavelength of the transmitted signal, all in feed. For a 1 degree beam width antenna, this distance is approximately 1400 feet.
In many cases, it is impractical to locate monitors in the far field; however, locating the monitor in the near field where the transmitted beam is unfocused can result in unacceptable performance, primarily due to phase errors and due to the magnification of the effects of system design tolerances. While somewhat acceptable results can be obtained with monitors located about 300 feet from the antenna, there is considerable room for improvement. Also, in some cases, even that distance is impractical and it would be desirable to be able to locate the monitor closer, say, perhaps as close as 80-90 feet and still be able to obtain acceptable results.
For antenna test ranges the far field distance of 2D.sup.2 /.lambda. is also the distance normally used in order to obtain accurate antenna measurements. Test signals radiated from a source at that distance arrive at an antenna under test in the form of an essentially plane wave front and such plane waves are well suited for antenna evaluation. However, where use of an indoor test range is desirable to avoid distortion of test results due to atmospheric effects or spurious signals from outside sources, or for security reasons, it may not be practical to provide for a radiating path length, between the source and the antenna under test, equal to the far field distance. Recognizing that test range measurement accuracy is typically required to exceed that which may be acceptable for operational monitoring of an antenna, test range operation with a radiating path length on the order of one-quarter of the far field distance or less would be desirable.
It is, therefore, an object of the present invention to provide a method and means for locating an antenna monitor in the near field relatively close to the antenna while obtaining acceptable monitoring.
It is an additional object of the invention to provide such means that may comprise only passive components, for reliability.
It is another object of the invention to provide such method and means that can be easily and simply applied without consuming a great deal of space.
Further objects are to provide signal sources and antenna test ranges permitting antenna performance evaluation based on near field signals.